In general, albumins are widely found in living material and known as simple proteins defined as yielding only alpha-amino acids and their derivatives on complete hydrolysis. They are mainly characterized by solubility in water and dilute aqueous salt solutions.
As simple proteins, albumins are not conjugated. Conjugated proteins are defined as containing a protein molecule bound to an organic, nonprotein prosthetic group.
Among those albumins isolated into crystalline form, serum albumin has been determined to possess a molecular weight of 65,000 to 69,000 daltons. The pI values range from 4.7 to 4.9. The molecular weight of bovine albumin on the basis of its amino acid sequence has been estimated at about 67,300 daltons. (See also The Plasma Proteins, F. W. Putnam, ed., Vol. 1, 1975, Academic Press).
Serum albumin is the principal serum protein of mammalian organism. In humans, the normal blood plasma range is 3.5-5.0 g of albumin/100 ml (or per dl). Albumin is also found in the extravascular spaces, the lymph, and in other biological fluids including e.g. amniotic fluid, bile, gastric juice, sweat and tears. Albumin is a major component of edema fluid. In fact, plasma albumin has been determined to have two major functions, one is maintenance of osmotic pressure, the other is transport of noncovalently bound substances. Such substances comprise metals and other ions, bilirubin, amino acids, fatty acids, enzymes, hormones and drugs. More recently, albumin has also been characterized in diabetic condition as capable of nonenzymatically binding several glucose molecules.
Albumin is produced substantially in the liver. In fact, liver cells or hepatocytes principally produce albumin. In a 70 kg human, the liver synthesizes about 12-14 g albumin per day. Albumin has a half life of about 20 days in man. Albumin synthesis comprises the classical intracellular assembly of 575 amino acids. Its secretion is inhibited by ouabain. Albumin synthesis is stimulated in vivo by cortisone and thyroid hormones (thyroxine and triiodothyronine). Albumin synthesis in vitro appears dependent on osmotic pressure such that it is increased under hypotonic and decreased under hypertonic conditions.
Due to its small size, albumin leaks through blood vessel walls into extravascular space to be returned to the blood circulatory system by the lymphatic system. In patients with kidney disease, albumin is known to pass into urine, frequently leading to hypoalbuminia.
It has been commonly accepted that serum albumin (or plasma albumin) is alone among major plasma proteins in that it contains no carbohydrate (Eylar, J. Theor. Biol. 19, 89, 1965).
However, nonenzymatic glycosylation or glycation of albumin in patients suffering from diabetes mellitus has been observed as the consequence of the condensation reaction between relatively high concentrations of glucose and the free amino group at the N-terminus or epsilon-amino group of lysine residues exposed on the albumin molecule. The reaction proceeds by Schiff base formation between carbonyl groups of glucose and the free amino groups. However, the nonenzymatic formation of a ketoamino is slow and concentration dependent. Thus prolonged hyperglycemia may increase the glycation level of albumin, in addition to that of many other serum proteins Moreover, the hitherto described glycated albumins appeared to contain only one sugar residue per binding site.
More recently, Robert J. Peach et al. (Biochimica et Biophysica Acta 1097 (1991) 49-54) characterized a new mutant form of albumin (Albumin Casebrook) carrying a single N-linked oligosaccharide which binds to Concanavalin A. The albumin variant constitutes about 35% of total serum albumin.
Surprisingly, a minor glycosylated albumin fraction has now been discovered as generally present in mammalian blood. The isolated protein exhibits a growth promoting and modulating effect in serum-free cultures of various cell types. While the biological role of this new circulating factor is still unknown, it has potential utility in the diagnosis and treatment of the processes of cell or tissue growth, development, differentiation and healing.